1. Field of the Invention
The invention relates to a polymer powder based on polyamide or on copolyamides, preferably nylon-12, which comprises phosphonate-containing flame retardant, to a process for producing this powder, and also to moldings produced by a layer-by-layer process which selectively melts regions or selectively binds them to one another, from this powder.
2. Discussion of the Background
Very recently, a requirement has arisen for the rapid production of prototypes. Selective laser sintering is a process particularly well suited to rapid prototyping. In this process, polymer powders in a chamber are selectively irradiated briefly with a laser beam, resulting in melting of the particles of powder on which the laser beam falls. The molten particles fuse and solidify again to give a solid mass. Three-dimensional bodies, including those of complex shape, can be produced simply and rapidly by this process, by repeatedly applying fresh layers and irradiating these.
The process of laser sintering (rapid prototyping) to realize moldings made from pulverulent polymers is described in detail in patent specifications U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers is claimed for this application, e.g. polyacetate, polypropylene, polyethylene, ionomers, and polyamide.
Nylon-12 powder (PA 12) has proven particularly successful in industry for laser sintering to produce moldings, in particular to produce engineering components. The parts manufactured from PA 12 powder meet the high requirements demanded with regard to mechanical loading, thus having properties particularly close to those of the mass-production parts subsequently produced by extrusion or injection molding.
A PA 12 powder with good suitability here has a median particle size (d50) of from 50 to 150 μm, and is obtained as in DE 19708946 or as in DE 4421454, for example. It is preferable here to use a nylon-12 powder whose melting point is from 185 to 189° C., whose enthalpy of fusion is 112 J/g, and whose freezing point is from 138 to 143° C., as described in EP 0911142.
Other processes with good suitability are the SIB process, as described in WO 01/38061, or a process as described in EP 1015214. The two processes operate using infrared heating over an area to melt the powder, and selectivity is achieved in the first process by applying an inhibitor, and in the second process by way of a mask. Another process which has found wide acceptance in the market is 3D printing, as in EP 0431924, where the moldings are produced by curing of a binder applied selectively to the powder layer. Another process is described in DE 10311438, in which the energy required for melting is introduced by way of a microwave generator, and selectivity is achieved by applying a susceptor.
For these processes, use may be made of pulverulent substrates, in particular polymers or copolymers, preferably selected from polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimide) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, copolyester, copolyamides, terpolymers, acrylonitrile-butadiene-styrene copolymers (ABS), or a mixture of these.
Although the known polymer powders intrinsically have good properties, moldings produced using these powders still have some disadvantages. A particular disadvantage with the polymer powders currently used is their high flammability and combustibility. This currently inhibits the use of processes described above in short runs in aircraft construction, for example.